Abstract: A method for determining real positions in a multi-leaf collimator (MLC) is disclosed. A scanner or other secondary imaging device is used to acquire a series of reference images and to capture the light field when the MLC is in a particular position and has a particular geometry. The reference images are used together with the captured light field images in order to determine the real positions of the leaves in the MLC. The real positions are used to correct any calibration problems in the mechanism used to drive the leaves of the MLC.